The invention related to a motor vehicle electrical system and, more particularly, to a motor vehicle electrical system having an electrical machine which can be operated in a first mode as an on-board power supply generator and, in a second mode, for supplying an electrical load which has a comparatively high power consumption.
In most motor vehicles, an on-board power supply generator is provided. The on-board power supply has an operating voltage regulated by a regulator through suitable adjustment of the generator excitation current. Furthermore, in most motor vehicles there are frequently electrical loads having a comparatively high power consumption, for example a heating device for an exhaust gas catalytic converter which can be electrically heated. The exhaust gas catalytic converter does not operate optimally until it reaches a specific operating temperature. In order to quickly reach this temperature during the starting phase of the motor vehicle engine, brief heating of the exhaust gas catalytic converter is frequently provided at high power, for example, 6 kW or more.
German patent document DE 42 30 597 C1 discloses a motor vehicle electrical system in the case of which the electricity for the electrical heating of the exhaust gas catalytic converter is produced by a separate generator which is exclusively provided for this purpose. The production of electricity is regulated by means of a controllable coupling between the generator and the internal combustion engine.
The German patent document DE 39 19 562 A1 discloses motor vehicle electrical systems in the case of which a common generator operates as an on-board power supply generator and is also used for supplying a heating resistor. In order to provide an increased generator voltage for supplying the heating resistor, various types of circuitry measures are disclosed. For example, the splitting of the generator operation is disclosed, which is provided in accordance with this generic type, into a first mode as an on-board power supply generator and into a second mode for a heating resistor supply.
The German patent document DE 24 41 086 discloses a speed-regulating V-belt drive which couples one or more units to the motor vehicle engine shaft. The coupling is in such a manner that the speed of the unit is reduced as the engine speed increases and the speed of the unit is increased as the engine speed reduces. This is accomplished by the V-belt being held on different pulley radii by centrifugal bodies.
The German patent document DE 37 43 317 A1 discloses a motor vehicle electrical system in the case of which an electrical machine can be operated on the one hand as an on-board power supply generator and on the other hand as a starter motor, for which purpose a suitable direct-current intermediate circuit is connected between the electrical machine and the vehicle battery as well as the on-board power supply loads.
There is therefore needed a motor vehicle system in the case of which, at a relatively low cost and with few components, both reliable on-board power supply and an at least brief supply of an electrical load having a comparatively high power consumption, especially a heating device for an exhaust gas catalytic converter, are ensured.
These needs are met according to the present invention by a motor vehicle system having an electrical machine which can be operated in a first mode as an on-board power supply generator and, in a second mode, for supplying an electrical load which has a comparatively high power consumption. A regulator sets the excitation current for the electrical machine. A controllable changeover switch is provided. A control unit controls the operation of the regulator and changeover switch optionally in the first mode or a second mode. In the first mode, the changeover switch connects the supply voltage output of the electrical machine to an on-board power supply cable. The regulator regulates the operating voltage of the electrical machine at a lower nominal value which is suitable for the on-board power supply. In the second mode, the changeover switch connects the supply voltage output of the electrical machine to the connection of the electrical load having a comparatively high power consumption. The regulator regulates the operating voltage of the electrical machine at a higher nominal value which is suitable for supplying power to this load. A coupling device, which couples the shaft of the electrical machine to a motor vehicle engine shaft is provided with a step-up ratio which is different in the two modes.
This solution provides an electrical machine which, on the one hand can carry out an on-board power supply generator function and, on the other hand, can supply the electrical load having a relatively high power consumption. For this purpose, the electrical machine can be suitably connected by means of the control unit, the changeover switch and the regulator, and can be mechanically coupled to the motor shaft via the drive coupling connection having a suitable step-up ratio in each case. Both the setting of a suitably increased generator excitation current and the selection of a higher step-up ratio, which results in an increased generator speed, contribute to achieving a higher generator output voltage for supplying the electrical load having a comparatively high power consumption. Consequently, there is no need for a separate electricity generator for the load having a comparatively high power consumption.
If the electrical machine is suitably designed and the operating point suitably selected then a considerably higher power than the rated power can in any case be taken, at an increased voltage, and made available to the load, for example an exhaust gas catalytic converter heater. This occurs briefly, by greater excitation. The associated cable cross-section and the losses occurring in the cable are kept low. For example, a generator having a rated power of 1.6 kW can briefly produce approximately 6 kW at a generator speed of 6000 rpm, an operating voltage of approximately 50 V being produced on the load. At a higher speed, the electrical power provided can be still further increased.
An advantageous embodiment of the drive coupling of the electrical machine to the motor vehicle engine shaft is also provided. This has the advantage that the achievement of a higher generator output voltage for supplying the electrical load having a comparatively high power consumption is brought about both by setting a suitably increased generator excitation current and by the activation of the step-up belt drive. By means of this, the electrical machine is mechanically coupled to the motor vehicle engine at a higher step-up ratio than when the normal belt drive is activated so that the generator speed which is increased in this way likewise contributes to the higher generator output voltage.
In a further embodiment of the invention, it is advantageous in design terms that the switching device be implemented by a friction locking coupling element to activate the step-up belt drive in the second mode and to deactivate it in the first mode. The friction-locking coupling element is designed in such a manner that, after the engine has been started, it produces a friction lock, for the purpose of the force-fitting connection. The friction lock is provided by means of the torque which acts from the engine shaft. The friction lock is automatically released on reaching a preselectable engine speed limit, for example by means of coupling element members which are operated by centrifugal force. The released connection and, thus, the deactivation of the friction-locking coupling element, are maintained during the further, running operation of the engine. This measure makes it possible for the electrical machine to be driven by the engine shaft, after the engine has been started, initially at a higher speed in the second mode, via the step-up belt drive, in order to provide a greater generator output power and, subsequently, at a lower speed in the first mode, via the normal belt drive, in order to provide the on-board power supply voltage. No external control of the friction-locking coupling element is required as a result of the automatic changeover in the friction-locking coupling element.
In a particularly advantageous embodiment of the present invention, the electrical machine can furthermore be operated in a third mode as a starter motor for starting the motor vehicle engine. In this case, the motor vehicle electrical system additionally has a current supply device for the armature circuit of the electrical machine. The control unit ensures that the regulator, the changeover switch, and this current supply device are suitably controlled. Consequently, it is initially possible to use the electrical machine in its third mode as a starter motor during starting of the motor vehicle and, once the running of the motor vehicle engine has been detected, then for a specific time period as a generator having a comparatively high output power and, subsequently, as an on-board power supply generator during the further, running operation of the motor vehicle.
The implementation of the third mode for the electrical machine as a starter motor is provided in a further embodiment which provides a third switching device in the form of a freewheel which, like the second switching device, acts on the step-up belt drive coupling. However, the free wheel acts in the opposite direction, that is to say it produces a force fit when the torque acts from the shaft of the electrical machine rather than from the engine shaft.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.